An optical disc such as a CD (Compact Disc), DVD (Digital Versatile Disc), or BD (Blu-ray Disc; registered trademark) is an optical information recording medium used for recording video data, audio data, or other information by irradiation with laser light and for reproducing the recorded information. Optical discs have increased in capacity through successive generations. For example, CDs achieve a 650 MB capacity with an approximately 1.2-mm thick light transmitting disc substrate layer, laser light having a wavelength of approximately 780 nm, and an objective lens having a numerical aperture (NA) of 0.45. After CDs came the generation of DVDs, which achieve a 4.7 GB capacity with an approximately 0.6-mm thick light transmitting disc substrate layer, laser light having a wavelength of approximately 650 nm, and an NA of 0.6. DVDs have, for example, a structure about 1.2 mm thick, with two disc substrates about 0.6-mm thick bonded together.
The standards for BDs, which have even higher recording densities, specify a protective (light transmitting) layer approximately 0.1 mm thick covering the information recording surface, laser light having a wavelength of approximately 405 nm, and a 0.85 NA. The capacities achieved are 25 GB for a single-layer BD having a single information recording surface, and 50 GB for a dual-layer disc having two information recording surfaces, enabling high-definition High-Vision video to be recorded in a BD over an extended time period.
It is anticipated that general users will be dealing with greatly increased amounts of data in next-generation high-definition video with higher resolution than High-Vision video, and in three-dimensional video, etc., so large capacity optical disc systems capable of storing larger amounts of data, exceeding the capacities of BDs, are being sought.
The above-described increasing capacities of optical discs have been achieved by shortening the wavelength of laser light and increasing the NA of an objective lens, thereby reducing both the size of the light spot focused on the focal plane of the objective lens and the size of the marks recorded on the tracks in the recording layer. In reducing the size of the focused spot, however, there is a physical limit defined by the optical characteristics of an objective lens and the wavelength of laser light. Specifically, the size limit of a reproducible recorded mark is said to be the diffraction limit λ/(4NA) determined by the wavelength λ of laser light and the NA of an objective lens.
In recent years, optical discs (referred to below as super-resolution optical discs) having a super-resolution functional layer with an optical characteristic (such as an optical absorption characteristic or optical transmission characteristic) that varies nonlinearly according to the intensity of the laser light are attracting attention as a way of achieving high-density recording and reproduction beyond the physical limits. When this super-resolution functional layer is irradiated with a focused spot of laser light, the refractive index or another optical characteristic changes in a local area of high light intensity or high temperature within the irradiated spot, and the diameter of the focused spot is reduced by this local area (referred to below as an aperture). This makes it possible to reproduce information from tiny recorded marks smaller than the λ/(4NA) diffraction limit by using recording/reproducing devices with conventional BD optical heads. By use of a super-resolution optical disc, accordingly, recording and reproducing data at recording densities higher than the BD recording density can be achieved even with, for example, a recording and reproducing device using laser light with a wavelength of 405 nm and an objective lens with a 0.85 NA.
There is a demand, however, for super-resolution optical discs with still higher recording densities and greater capacities. It would therefore be desirable to select better materials for use in the super-resolution functional layers of super-resolution optical discs, and to optimize the layer structure and recording marks of super-resolution optical discs.
Japanese Patent No. 3866016 (patent reference 1), Japanese Patent Application Publication No. 2007-506219 (Japanese translation of PCT patent application, patent reference 2), and Japanese Patent Application Publication No. 2009-37698 may be cited as prior art references related to super-resolution optical discs. A method of measuring the optical characteristics of a super-resolution optical disc is disclosed in non-patent reference 1, listed below.
Optical discs having a super-resolution structure are disclosed in patent references 1 and 2. The pit depths of the super-resolution pits (non-flat optical recording marks smaller than the diffraction limit) in these optical discs are made shallower than the pit depths of non-super-resolution pits (non-flat optical recording marks larger than the diffraction limit). The modulation amount of the reproduced signal can thereby be increased.
An optical information recording medium having a super-resolution layer is disclosed in patent reference 3. In order to secure adequate reproduction performance with reduced thermal load on the medium, all of the pit depths in this optical information recording medium are restricted to the range from λ/(10n) to λ/(6n). Here, λ indicates the wavelength of the laser light and n indicates the refractive index of the substrate through which the laser light passes.